Here's my 2 cents, and let me preface it by saying I've only raced in local autocrosses at my former Univeristy in SC about a dozen times or so. I was running on Comp t/a ZR 225/60-15. I didn't have any other vintage mustangs to run against, but I did usually run better than 90% of the other late model mustangs there.
I run the setup below, and despite Historic's suggestion not to run a rear bar, I've found even with the 3/4 rear bar, I can bring the back end of the car around fairly easily when I'm not paying attention, or need to. The rear bar does seem to help plant the rear earlier leaving the corner, allowing me to accelerate sooner. This setup seems to work ok, except on the tightest 180's, but that's probably due to still using the original Non-ps box. I'd like to get a faster ratio flaming river box. With the '71 spindles, I think the steering effort has decreased since the tie rod arm is slightly longer than the '65, by how much I don't know.
An aside on wheels, '65-66 will take a 235/60 on 15x8 with 5" BS in the back, and 225/60 on 15/7 w 4" BS in the front quite nicely. The 235 looks w-i-d-e on such a small car. Diamond Racing can make some spun steel rims in just about any size and BS you need, they only run 21 lb for a 16x7, about the lightest steel wheel you'll find.
As someone else said, more brakes is almost never overkill, especially in the brake fade department.
In my opinion, you could build a car capable of great things, but if you aren't ready to drive the hairy beast it'll probably be, you won't be having as much fun as you'd like.
On the technical side, most authors I've read say all that can be done for vintage racers is lots of spring and sway bar to be legal. If all you are going to want to do is race for fun, then stiffen the chassis considerably and you can use less spring and sway bar, making the car more livable on rough(ie., normal road) surfaces.
ya, ya I talk too much...
